Optical fiber laser, and components for an optical fiber laser, having reduced susceptibility to catastrophic failure under high power operation

ABSTRACT

Optical fiber lasers and components for optical fiber laser. An optical fiber laser can comprise a fiber laser cavity having a wavelength of operation at which the cavity provides output light, the cavity including optical fiber that guides light having the wavelength of operation, the fiber having first and second lengths, the first length having a core having a V-number at the wavelength of operation and a numerical aperture, the second length having a core that is multimode at the wavelength of operation and that has a V-number that is greater than the V-number of the core of the first length optical fiber at the wavelength of operation and a numerical aperture that is less than the numerical aperture of the core of the first length of optical fiber. At least one of the lengths comprises an active material that can provide light having the wavelength of operation via stimulated emission responsive to the optical fiber receiving pump light. Components include a mode field adapter and optical fiber interconnection apparatus, which can be used to couple the first and second lengths of optical fiber, or can couple the fiber laser to an optical fiber power amplifier, which can be a multimode or single mode amplifier.

FIELD OF THE INVENTION

The present invention relates to optical fiber lasers and optical fibercomponents therefore, having reduced susceptibility to catastrophicfailure under high power operation.

BACKGROUND

Optical fiber devices, such as optical fiber lasers and amplifiers, thatcan provide high output power as well as high beam quality, are ofconsiderable commercial and academic interest. For example, opticalfiber lasers that provide a low M² (a measure of beam quality) andcontinuous wave (CW) output powers in the range of tens to hundreds oreven thousands of Watts have many industrial applications, as do pulsedlasers having good beam quality and high peak powers. In a fiber lasershaving a “master oscillator-power amplifier” (MOPA) architecture, a lowpower laser source (the “master oscillator”) provides an input “seed”beam to an optical fiber amplifier (the “power amplifier”) thatamplifies the seed beam. Unfortunately, nonlinear phenomena, such asStimulated Raman Scattering (SRS) or Stimulated Brillouin Scattering(SBS), can severely limit scaling the output power of such a fiber laserto higher powers. Furthermore, avoiding nonlinear phenomena as well asmaintaining good beam quality can be difficult. Good beam qualitytypically requires single transverse mode operation of the MOPA, whichin turn typically requires optical fibers having cores of relativelysmall cross sectional area. However, a small cross sectional arearesults in a higher power density, and high power density more readilytriggers nonlinear effects. Increasing the cross sectional are of thecore lowers the power density, and helps avoid nonlinearities, but coreshaving larger cross sectional areas are typically multimode, which tendsto degrade the beam quality and hence raise the M² parameter.

U.S. Pat. No. 5,818,630 teaches one approach for maintaining good beamquality in fibers having cores having larger cross sectional areas. The'630 patent teaches a mode converter that receives an input beam from alaser seed source having a nearly diffraction limited mode. The modeconverter converts the mode of the input beam to match a fundamentalmode of a multimode fiber amplifier, providing a mode converted inputbeam to the multimode fiber amplifier. Because the optical energydelivered to the fiber amplifier is matched to the fundamental mode, theamplifier, despite being a multimode amplifier, provides at an outputthereof an amplified beam in the fundamental mode. Keeping the multimodefiber of the multimode amplifier as straight as possible helps avoidmode coupling between the fundamental and higher order modes. Modecoupling is undesirable, as energy transferred to higher order modeswould then be amplified by the multimode amplifier, which would degradebeam quality. However, the '630 patent indicates that coiling of thefiber can be tolerated without incurring detrimental mode coupling.

It is an object of the present invention to provide improved methods andapparatus for proving higher optical powers from optical fiber devicessuch as optical fiber lasers and amplifiers.

SUMMARY OF THE INVENTION

In investigating potential designs for a high power optical fiber MOPAapparatus, Applicant initially pursued a design using a commerciallyavailable mode field adapter interposed in the optical path between thesingle mode output fiber of a master oscillator and the multimode inputoptical fiber of an optical fiber amplifier. Applicant learned that themode field adapter can suffer catastrophic failures during operation ofsuch high power optical fiber MOPA apparatus. Post mortem examination ofthe mode field adapter revealed optical fiber simply missing, havingapparently exploded and/or vaporized.

The catastrophic destruction is considered to be due to a burst of“backward propagating” optical energy, that is, optical energyapparently originating in, or at least being amplified by, the poweramplifier and propagating backwards towards the master oscillator ratherthan out of the output of the power amplifier. The backward propagatingenergy can be a sudden and unpredictable pulse. The phenomenon has arogue nature, akin to the destructive, rogue ocean waves known tosuddenly appear and swamp ships on the open sea. The likely mechanismsthat create the backward propagating pulse are generally understood.Such rogue pulses generally result when the gain of the amplifier, whichis function of the inversion of the active material of the amplifier andother factors, such as spurious feedback, exceeds the losses of theamplifier. Rogue pulses are thus more likely under high inversion, whichincreases gain. A rogue, backward propagating pulse can be triggered ina high gain system by excessive feedback at the output of the poweramplifier. A rogue pulse could occur upon temporary failure of a masteroscillator (MO) to provide a seed signal, as the lack of seed signalwould lead to high inversion. In a pulsed system, a long mark spacebetween two pulses can also create high inversion, leading to a roguepulse. Although rogue pulses can be caused by a MO failure, perhapscausing a mode field adapter to fail, it is understood that they couldalso be the cause of MO failure.

Accordingly, the invention can provide more robust apparatus that canreduce the susceptibility of optical fiber lasers or amplifiers todamage. For example, in one aspect of the invention, there is taught amore robust mode field adapter that is considered less susceptible tocatastrophic destruction, as well as, in other aspects, designs andarrangements of fiber based components for making a more robust fiberlaser or amplifier. There are many aspects of the invention, which canbe practiced alone or in combination. Certain aspects of the inventionare described in more detail below.

In one aspect, the invention provides an optical fiber apparatus forhaving an increased optical power threshold for avoiding damage to theapparatus. The apparatus can comprise an optical fiber mode fieldadapter having an input and an output, the mode field adaptertransforming optical energy from a fundamental mode having a smallermode field diameter at the input to a fundamental mode having largermode field diameter at the output. The mode field adapter can include alength of optical fiber comprising a core including a taper wherein thecross sectional area of the core increases; a cladding disposed aboutthe core for tending to confine light to the core so as to be guided bythe core; a region disposed about the cladding, the region comprising amaterial contactingly disposed about the cladding and having an index ofrefraction that is greater than an index of refraction of the claddingby a selected amount, the region stripping optical energy from thecladding, the selected difference no greater than 0.035; and a secondregion disposed about and in optical communication with the region, thesecond region for one or reflecting or absorbing optical energy strippedfrom said cladding by said region.

The second region can comprise a metal, such as, for example, metalparticles within a matrix material. The second region can comprise asolid metal structure, such as an aluminum structure, contacting theregion. In various practices of the invention, the aforementioneddifference can be no greater than 0.03; no greater than 0.028; nogreater than 0.026; no greater than 0.024; no greater than 0.022, or nogreater than 0.02. The taper can comprise an adiabatic taper. The modefield adapter can have a wavelength of operation wherein at thewavelength of operation the mode field adapter is single mode at theinput and multimode at the output. The mode field adapter can have awavelength of operation, and the material comprised by the region canhave an optical transparency at the wavelength of operation of at least85%. The optical transparency can be at least 95% or even 100%, asmeasured by UV/VIS spectroscopy. The cladding can include a taperwherein an outer diameter of the cladding tapers from a smaller diameterto a larger diameter. The cladding of the length of fiber can besubstantially free of a taper, meaning that an outer diameter of thecladding remains substantially constant.

The input of the mode field adapter can comprise a length of inputoptical fiber comprising a core and the output can comprise a length ofoutput optical fiber comprising a core, and the core of the inputoptical fiber can have a first numerical aperture and the core of theoutput optical fiber can have a numerical aperture that is less than thefirst numerical aperture. The input and output each comprise arespective length of optical fiber comprising a core, a claddingdisposed about the core and a selected region contactingly disposedabout the cladding, where the selected region comprises an index ofrefraction that is different by a predetermined amount than an index ofrefraction of the cladding and wherein the predetermined amountcorresponding to the input length of fiber and the predetermined amountcorresponding to the output length of fiber are both greater inmagnitude than the magnitude of the selected difference. The magnitudeof the predetermined amount corresponding to the input length of opticalfiber can be different than the magnitude of the predetermined amountcorresponding to the output length of optical fiber.

In one aspect, the invention provides an optical fiber interconnectionapparatus for optically coupling a source of optical energy having anoutput wavelength and an optical amplifier and having an increased powerthreshold for avoiding damage to the source or interconnection apparatusfrom optical energy propagating back from the optical amplifier to thesource. The optical fiber interconnection apparatus can comprise anoptical fiber arrangement having an input reference plane and an outputreference plane and at least one intermediate section of optical fiberhaving a core spliced at first and second ends, respectively, to thecores of first and second sections of optical fiber. The optical fiberinterconnection apparatus can include a first geometrical mismatch and afirst numerical aperture mismatch between the core of the intermediateoptical fiber and the core of the first section of optical fiber and asecond geometrical mismatch and a second numerical aperture mismatchbetween the core of the intermediate section and the core of the thirdsection of optical fiber. The mismatches can be such that in onedirection of propagation, referred to as the forward propagationdirection wherein optical energy propagates from an input referenceplane to the output reference plane, the geometrical mismatches are fromsmaller cross sectional area cores to larger cross sectional area coresand the numerical aperture mismatches are from larger numerical aperturecores to smaller numerical aperture cores. In the opposite direction ofpropagation, wherein optical energy propagates from the output referenceplane to the input reference plane, the geometrical mismatches are fromlarger cross sectional area cores to smaller cross sectional area coresand the numerical aperture mismatches are from smaller numericalaperture cores to larger numerical aperture cores.

The magnitude of the first geometrical mismatch can be greater than themagnitude of the second geometrical mismatch. The first, second andintermediate sections of optical fiber can each include a respectivecladding disposed about their respective cores for tending to confinelight to each of the cores and there can be a third geometrical mismatchbetween the cladding of the first section and the cladding of theintermediate section and a fourth geometrical mismatch between thecladding of the intermediate section and the cladding of the secondsection. The magnitude of the fourth geometrical mismatch can be greaterthan the magnitude of the third geometrical mismatch. One of the coresto which the core of the intermediate section is spliced can be singlemode at a wavelength selected from the range of 1 to 3 microns (or fromthe range of 1 to 2.2 microns) and the other of the cores is multimodeat the selected wavelength. The core of the intermediate section can bemultimode at the selected wavelength. Optical fiber of the fiberarrangement can include a cladding disposed about the core of theoptical fiber for tending to confine light to the core such that thecore guides light and a region contactingly disposed about a length ofthe cladding, where the region having an index of refraction that ishigher than an index of refraction of the cladding for stripping lightfrom the cladding. The region can be disposed about one of the splicesof the optical fiber interconnection apparatus. In various practices ofthe invention the difference between the index of refraction of theregion and the index of refraction of the cladding is no greater that0.035, no greater than 0.030, no greater than 0.028, no greater than0.026, no greater than 0.024, no greater than 0.022, or no greater than0.020. The optical fiber arrangement can include a coating contactinglydisposed about a different length of the cladding, the coating having anindex of refraction that is higher than the index of refraction of theregion. A second region can be contactingly disposed about and inoptical communication with the region, and the second region cancomprise a metal for absorbing light stripped from the cladding by theregion.

In another aspect, the invention can provide an optical fiberinterconnection apparatus for optically coupling a source of opticalenergy having a selected wavelength and an optical fiber amplifier andhaving an increased power threshold for avoiding damage to the source orinterconnection apparatus from optical energy propagating back from theoptical fiber amplifier to the source. The optical fiber interconnectionapparatus can comprise an optical fiber arrangement having an inputreference location along an input optical fiber and an output referencelocation along an output optical fiber, the arrangement comprising atleast one intermediate section of optical fiber spliced at first andsecond ends to other optical fibers, there being a geometrical mismatchbetween the core of the intermediate optical fiber and the cores of eachof the other optical fibers. In one direction of propagation, referredto as the forward propagation direction, optical energy propagates fromthe input reference location to the output reference location and thegeometrical mismatches are from smaller cross sectional area to largercross sectional area cores and in the opposite direction of propagation,referred to as the reverse propagation, the geometrical mismatches arefrom larger cross sectional area to smaller cross section area cores.The optical insertion loss of the optical fiber arrangement can be nonreciprocal in that the insertion loss in the reverse propagationdirection is substantially higher than the insertion loss in the forwardpropagation direction at a wavelength of operation of the optical fiberinterconnection apparatus.

In a further aspect, the invention can provide an optical fiber laserapparatus. The optical fiber laser apparatus can comprise a fiber lasercavity having a wavelength of operation at which the fiber laser cavityprovides output light. The fiber laser cavity can include optical fiberthat guides light having the wavelength of operation and the opticalfiber can have first and second lengths. The first length of opticalfiber can have a core having a V-number at the wavelength of operationand a numerical aperture and the second length of optical fiber can havea core that is multimode at the wavelength of operation, a V-number thatis greater than the V-number of the first length of optical fiber at thewavelength of operation and a numerical aperture that is less than thenumerical aperture of the core of the first length of optical fiber. Atleast one of the lengths can comprise an active material that canprovide light having the wavelength of operation via stimulated emissionresponsive to the fiber receiving pump light.

The fiber laser cavity can comprise a reflector that can reflect lighthaving the wavelength of operation and transmit light having thewavelength of operation for providing an output from the fiber lasercavity. The second length of optical fiber can comprise the reflector.The reflector can comprise a fiber Bragg grating. The core of the secondlength of fiber can have a numerical aperture that is no greater than0.10, no greater than 0.09, no greater than 0.08, no greater than 0.07,no greater than 0.06, or no greater than 0.05. The core of the firstlength of optical fiber can be single mode at the wavelength ofoperation. The first length of optical fiber can be spliced to thesecond length of optical fiber.

The fiber laser apparatus can comprise a third length of optical fiberoutside of the fiber laser cavity, where the third length of opticalfiber has a multimode core for propagating the output light from thefiber laser cavity, and the fiber laser apparatus can include an opticalfiber amplifier in optical communication with the third length ofoptical fiber. The optical fiber amplifier can comprise optical fibercomprising a core comprising active material for amplifying opticalenergy having the output wavelength responsive to receiving pump light.The optical fiber of the amplifier can have a core that is normallymultimode at the operating wavelength, and the fiber can be coiled suchthat the optical amplifier is a single mode amplifier.

The optical fiber comprised by the optical fiber amplifier can include acore having a diameter and a numerical aperture that are, respectively,substantially the same as a diameter and the numerical aperturecomprised by the core of the third length of fiber. The fiber lasercavity can be substantially free of any optical fiber having a core thatis multimode at the wavelength of operation and that comprises theactive material. The fiber laser cavity can comprise a mode fieldadapter arranged so as to provide optical communication between saidfirst and second lengths of optical fiber, said mode field adaptercomprising an optical fiber comprising a core having a taper.

The optical fiber laser cavity can comprise an optical fiber arrangementfor providing optical communication between the first and second lengthsof optical fiber. The optical fiber arrangement can comprise at leastone intermediate section of optical fiber having a core spliced at firstand second ends, respectively, to the cores of first and second sectionsof optical fiber. There can be a first geometrical mismatch and a firstnumerical aperture mismatch between the core of the intermediate opticalfiber and the core of the first section of fiber and a secondgeometrical mismatch and a second numerical aperture mismatch betweenthe core of the intermediate section and the core of the third sectionof fiber. The mismatches can be such that in one propagation directionfor optical energy by the optical fiber arrangement the geometricalmismatches are from smaller cross sectional area cores to larger crosssectional area cores and the numerical aperture mismatches are fromlarger numerical apertures to smaller numerical apertures.

In yet a further aspect, the invention can provide an optical fiberlaser having first and second fiber Bragg gratings for reflecting lighthaving a selected wavelength therebetween so as to define a fiber lasercavity. The fiber laser cavity can comprise optical fiber and a selectedlength of the optical fiber can comprise a rare earth for providingoptical gain such that the fiber laser cavity provides output lighthaving the selected wavelength responsive to the cavity receiving pumplight. The optical fiber of the fiber laser cavity can comprise bothmultimode and single mode optical fiber arranged for guiding lighthaving the selected wavelength and that is reflected by the fiber Bragggratings, where the terms “single mode” and “multimode” are used withreference to the core of the optical fiber at the selected wavelength.The first fiber Bragg grating can be comprised by single mode fiber andthe second fiber Bragg grating can be comprised by multimode fiber. Thesecond fiber Bragg grating can have a selected transmissivity at theselected wavelength for providing the output light from the fiber lasercavity and the fiber laser apparatus can comprise a length of multimodefiber delivering the output light from the fiber laser cavity.

At least a majority of the optical gain can occur within single modefiber of the fiber laser cavity. The fiber laser cavity can besubstantially free of any multimode optical fiber that comprises therare earth. The fiber laser apparatus can include a coupler for couplingpump light to the fiber laser cavity, and the coupling can be achievedwithin the fiber laser cavity. In one practice, the coupler couples pumplight substantially only to single mode fiber of the fiber laser cavity.The fiber laser cavity can comprise single mode fiber having a firstnumerical aperture and multimode fiber a having a core having a secondnumerical aperture that is less than said first numerical aperture. Invarious practices of the invention, the second numerical aperture is nogreater than 0.11, no greater than 0.10, no greater than 0.09, nogreater than 0.08, no greater than 0.07, no greater than 0.06, or nogreater than 0.05. The fiber laser cavity can comprise large mode area(LMA) multimode fiber.

The optical fiber of the optical fiber laser cavity can comprise asection having a core, a cladding disposed about the core and a materialcontactingly disposed about the cladding and having an index ofrefraction that is higher than an index of refraction of the cladding bya selected amount. In one practice, the amount is no greater than 0.035.In other practices of the invention, the selected amount can be nogreater than 0.030, no greater than 0.028, no greater than 0.026, nogreater than 0.024, no greater than 0.022, or no greater than 0.020. Thematerial can selectively strip light from said cladding. The fiber laserapparatus can include an optically absorbing region contactinglydisposed about said material for absorbing the stripped light. The fiberlaser cavity can comprise multimode double clad fiber.

Certain terms are used above and elsewhere herein. Stating herein that afirst region is disposed about a second region means that the firstregion surrounds, at least partially, the second region, and may or maynot contact the second region, as “disposed about” does not preclude theexistence of an intermediate region or regions interposed between thefirst region and the second region. A splice, such as, for example, afusion splice, is well known in the fiber optic art. A fusion splice canbe accomplished by the application of localized heat sufficient to fuseor melt the ends of two lengths of optical fiber. A fusion splicingapparatus that uses an electrical arc discharge to provide localizedheating is commercially available from companies such as Fujikura andSumitomo, both of Japan. Fusion splices between identical and alignedfibers are often possible wherein the splice loss is rather low (e.g.not greater than 0.1 dB) at an operating wavelength or over an operatingrange of wavelengths of the spliced optical fibers. The term “splice,”as used herein, is not limited to fusion splices, and includes, forexample, fibers that are butt coupled together using, for example, anadhesive at the region of the joint and/or mechanical means (e.g.,threaded connectors), to secure the butt joint together. The butt jointcan, but preferably does not, include a small gap between the ends ofthe butt coupled fibers.

“Optical fiber” is not to be taken as limited to a continuous,undisturbed length of optical fiber whose characteristics aresubstantially longitudinally unvarying, such as would be formed in aconventional well run draw and spooling procedure using a preformmounted on a draw tower, but can include, for example, similar ordissimilar sections of fiber spliced together. “End,” as used whenreferring to an “end” of an optical fiber, is not limited to aphysically free end, but can include a location along a fiber, such aswherein one particular fiber is joined to what was initially a separatefiber. For example, a fiber can have an “end” at a splice. A “length” or“section” of optical fiber can refer to a distance between referenceplanes. The term “light” or “optical energy”, as used herein, means, asunderstood by one of ordinary skill in the optical arts, theelectromagnetic energy associated with the optical apparatus inquestion, and is not to be limited to, for example, wavelengths visibleto the human eye, which is a definition that can be found in certaindictionaries intended for laypersons. Ranges stated herein, such as ofwavelengths, include the endpoints (e.g., between 1 micron to 2 micronsincludes 1 and 2 microns). The terms “first,” “second,” “third,” etc.are used as convenient designators that more readily allow for clearantecedent basis for subsequent references to, for example, a region orsection of a fiber. For example, stating that a first length of fiberhas a first core and a second length of fiber has a second core shouldnot, taken alone, be taken to mean that the second length of fiber musthave two (first and second) cores simply because reference was made to asecond core of the second fiber. A “region” need not be homogeneousthroughout and can be comprised, for example, of more than one layer.One region being “contactingly disposed” about another means in suchphysical proximity such that any intervening substance or material hasno substantial detrimental effect on the intended primary functionalrelationship, (e.g., light stripping) between the regions.

The multimode power amplifiers in MOPA configurations are often based onoptical fibers generally referred to as Large Mode Area (LMA) fibers,though to Applicant's current knowledge there is not a specific,universal definition of the term “LMA” in the art. An LMA fiber typicalincludes both a larger core diameter (typically greater than 10 microns)as well as a lower numerical aperture (NA) than a single mode fiber. Asone of ordinary skill in the art understands, the NA of a fiber isrelated to the angle of the cone of rays that can be guided by thefiber, such as by total internal reflection, and for a conventional stepindex fiber can be calculated as the square root of the difference ofthe squares of the refractive index of the core and the refractive indexof the cladding. The lower NA of the fiber increases the Mode FieldDiameter (MFD) of the fundamental mode fiber, which reduces the powerdensity so as to avoid nonlinearities, as well as lowers the V-number,such that the fiber, while typically multimode at the wavelength ofoperation of the MOPA, supports fewer modes than it would if it had alarger NA, helping preserve beam quality. The term multimode, as usedherein, means that the region of fiber of interest, typically the core,can support more than just the fundamental mode (i.e., can supporttransverse modes in addition to the fundamental, or LP₀₁, mode at theoperational wavelength of interest). The operational wavelength ofinterest is typically the desired wavelength at which the laserapparatus provides light responsive to the optical pumping, which can bedetermined, for example, by the gain and absorption spectra of theactive material and the resonance wavelength of the fiber laser cavity.For example, a core that can support both LP₀₁ and LP₁₁ is consideredmultimode. V-number, for a round core, step index fiber or effectiveequivalent, can be defined as (Diameter of core×NA ofcore×π)/(wavelength of operation). A V-number greater than 2.405typically means that the core of the fiber can support more than onetransverse mode, meaning that the core is multimode. Higher V-numberfibers support more higher order modes.

Further advantages, novel features, and objects of the invention willbecome apparent from the following detailed description of non-limitingembodiments of the invention when considered in conjunction with theaccompanying FIGURES, which are schematic and which are not necessarilydrawn to scale. For purposes of clarity, not every component is labeledin every one of the following FIGURES, nor is every component of eachembodiment of the invention shown where illustration is not considerednecessary to allow those of ordinary skill in the art to understand theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates one example of a prior art opticalfiber master oscillator-power amplifier (MOPA) apparatus;

FIG. 2 schematically illustrates Applicant's understanding of a priorart mode field adapter;

FIG. 3 schematically illustrates one embodiment according to theinvention of an improved optical fiber coupling apparatus for providingdecreased susceptibility to catastrophic damage to a source or to theoptical fiber coupling apparatus from rogue optical energy propagatingback from the optical fiber amplifier to the source;

FIG. 4 schematically illustrates an alternative embodiment according tothe invention of an optical fiber coupling apparatus for providingdecreased susceptibility to catastrophic damage to a source or to theoptical fiber coupling apparatus from rogue optical energy propagatingback from the optical fiber amplifier to the source;

FIG. 5 schematically illustrates one embodiment of an optical fibermaster oscillator-power amplifier apparatus that includes the opticalfiber coupling apparatus of FIG. 4; and

FIG. 6 schematically illustrates another embodiment of an optical fibermaster oscillator-power amplifier apparatus according to the invention.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates one example of a prior art opticalfiber MOPA apparatus 12. The MO 14 can comprise, for example, a singlemode (SM) fiber laser that includes a pair of Fiber Bragg Gratings(FBGs) that define a laser cavity for generating the output of the MO14. The length of SM fiber 16 delivers the output from the laser cavityto the mode converter 18. Accordingly, the MO 14 can provide a SM (e.g.,diffraction limited) input beam along the SM fiber 16 to the modeconverter 18. The input beam is thus in the fundamental mode and has asmall diameter modal profile. The mode converter 18 receives the inputbeam and converts the fundamental mode of the input beam to match themodal profile of the fundamental mode of the optical fiber multimode(MM) power amplifier 24. The fundamental mode of the MM power amplifier24 has a different modal profile, such as by at least having a largerdiameter than the modal profile of the SM input beam delivered by fiber16 to the mode converter 18. The mode converter 18 thus converts themodal profile of the input beam to match the modal profile of thefundamental mode of the optical MM amplifier and delivers, via MMoptical fiber 22, the mode converted beam to the MM power amplifier 24.Because of the mode matching to the fundamental mode, the MM poweramplifier 24, despite being capable of amplifying multiple modes, isexcited only in the fundamental mode (fundamental mode as used hereinrefers to LP₀₁), and hence can provide an output only in the fundamentalmode along MM optical fiber 28, thus providing an output beam 32 havinggood beam quality. The MO 14 and MM power amplifier 24 can each includea pump source for pumping the active media, respectively, of the MO 14and the MM power amplifier 24. The active media typically comprises arare earth, such as, for example, ytterbium.

As noted above in the Summary, Applicant found that commercial modefield adapters interposed between MO seed sources and optical amplifierscan suffer catastrophic failures during operation of the high poweroptical fiber MOPA apparatus. Optical fiber of the mode field adaptervirtually ceases to exist, apparently exploding or vaporizing, andapparently due to a burst of “backward propagating” optical energy.“Backward propagating” optical energy refers to optical energyapparently originating in, or at least being amplified by, the poweramplifier and propagating backwards towards the MO rather in the otherdirection, as is normal, and out of the output of the power amplifier.Accordingly, Applicant has considered the damage mechanisms anddiscloses optical apparatus, and methods for making and using suchapparatus, to reduce susceptibility to damage from such rogue pulsessuch that when they do occur the optical apparatus can more readilysurvive.

Without wishing to be bound by theory, Applicant's understanding of thestructure and operation of a mode field adapter is now discussed alongwith the likely damage mechanisms, as a preface to Applicant's teachingof more robust optical apparatus having reduced susceptibility to damagefrom rogue, backward propagating pulses.

A fiber optic mode field adapter can use a fiber taper, in which a fiberis melted and stretched so that its diameter (and therefore core size)varies continuously over a length understood to be at least severalmillimeters (several thousands of wavelengths). The core physicallytapers from having a smaller cross section, and hence a smaller modalprofile for the fundamental mode, to a larger cross section, and hence alarger modal profile. The mode field adapter can be interposed betweentwo SM optical fibers having different modal profiles to decrease theinsertion loss over just splicing the two fibers together. Matching themodal profile is important because is well known that any mismatchbetween the MFDs of fundamental modes (such as in a splice between SMfiber having different modal profiles) increases the insertion lossconsiderably for transmission between the fundamental modes. Thetransmission factor, T_(FM), as a result of MFD mismatch in thefundamental mode is given by the formula:

T _(FM)=4/[(MFD ₁ /MFD ₂)+(MFD ₂ /MFD ₁)]²

Where MFD₁ is the mode field diameter of one of the fundamental modesand MFD₂ is the mode field diameter of the other fundamental mode. Forequal MFDs, the transmission is 1; however, for unmatched, or unequalMFDs, the transmission factor is less than 1 and decreases quickly asmismatch between the MFDs increases.

In most MOPA configurations, it is desired that the mode field adapterconvert the fundamental modes, not between SM fibers, but between a SMfiber and a MM fiber so as to match the modal profile of the fundamentalmode of the SM fiber to that of the MM optical fiber. Here matching iscritical, because energy not transferred between fundamental modes doesnot merely increase insertion loss, but will likely excite higher ordermodes in the MM optical fiber. As the MM optical amplifier is quitecapable of amplifying higher order modes (by definition it is a“multimode” amplifier), any excitation of the higher order modes willresult in an output beam that includes higher order modes and hence isdetrimental to beam quality (raises M²). However, when the fiber taperis carefully made, including being sufficiently gradual, the modalprofile of the fundamental mode gradually expands in size withouttransferring energy out of the fundamental mode (e.g., into lossy leakymodes or into higher order modes), such that the optical energycurrently confined at one end of the taper to the size of the LP₀₁ modeof a SM fiber is converted along taper, as the taper increases, to aincreasing modal profile. A gradual taper avoids a “mismatch” along thetaper. When the profile matches the desired profile of the fundamentalmode of the MM fiber, the taper must stop to avoid a mismatch with theMM fiber to which the mode converter matches, as that would also excitehigher order modes (this is presumably done in practice by starting withthe MM fiber and tapering it down to a SM fiber). Thus a mode fieldadapter designed to convert one modal profile to match another, such asmatching the modal profile of a SM optical fiber to a MM optical fiber,can have two advantageous features—very low insertion loss and noexcitation of higher order modes in the MM optical fiber. Analogiesexist in the electrical field—a transformer, for example, transforms theimpedance of a load to match that of a source so as to ensure that allthe power of the source is delivered to the load. As another example, inantenna theory, the spacing of a parallel-conductor transmission linecan be tapered gradually over a long distance for the purpose ofimpedance matching an RF signal generator to an antenna.

FIG. 2 schematically illustrates Applicant's understanding of a priorart mode field adapter 42. The mode field adapter 42 comprises a SMinput fiber section 44, a MM output optical fiber section 46, and atapered length of fiber 48 between the SM input fiber 44 and the MMoutput optical fiber section 46. The input optical fiber section 44includes a SM core 50 and a silica glass cladding 52 surrounding the SMcore 50, as well as a polymer coating 56 surrounding the silica glasscladding 52. The polymer coating 56 of the input optical fiber section44 typically has a higher index of refraction than the silica glasscladding 52. The output optical fiber section 46 is typically a doubleclad fiber including a MM core 60, an inner cladding, or “pump,”cladding 62 surrounding the core 60, and a polymer coating 66surrounding the inner cladding 62. The polymer coatings 56 and 66 areunderstood to likely be removed along the tapered section 48, whichincludes the tapered core 68 and tapered cladding 70, and perhaps aswell as along adjacent portions of the input and output fiber sections,as indicated in FIG. 2. Either no recoating is understood to be present,such that the mode converter is “air” clad along the section indicatedby reference numeral 72, or a perhaps a low index recoating, having anindex of refraction that is less than the index of refraction of thetapered cladding 70 and which would cause light to tend to be confinedin the cladding, may be used. As noted above, in practice, the modefield adapter is most likely fabricated by tapering the larger outputfiber section 46 to reduce the diameter of the core to the point wherethe MFD becomes close to, or if desired for mode conversion, matches theMFD of the SM input fiber section 44, and the SM input fiber section 44and tapered end are spliced together, such as at the location indicatedby reference numeral 74. Other techniques may be possible, such as, forexample, specially designing the input fiber section 44 such the thatthe core can be gradually expanded as a function of length using thermaltechniques to make a section akin to section 72 wherein the diameter ofthe core increases as shown in FIG. 2. In a fiber having such athermally expanded core (TEC), the outside diameter of the cladding maynot be not tapered. With reference to FIG. 1, it is understood that amode converter that matches the MFDs can provide nearly 100% of theoptical power from the fundamental mode of the SM optical fiber 16 tothe fundamental mode of the MM optical fiber 22.

Applicant purchased commercially available mode field adapters from ITFLabs, 400 Montpellier Boulevard, Montreal, Quebec H4N 2G7, Canada,understood to be equivalent to current model number MFA100S2021 (fromwhich the following specifications are noted). The purchased mode fieldadapters had an operating wavelength range of 1040-1080 nm and includedCorning™ HI 1060 input optical fiber, having, to the best of Applicant'sunderstanding, a core having a diameter of approximately 6 microns, asilica glass cladding having a diameter of 125 microns, and polymercoating disposed about the silica glass cladding. The core is understoodto have a numerical aperture of 0.15 relative to the cladding, thepolymer coating is understood to have an index of refraction of about1.5, as opposed to a refractive index of about 1.452 for the silicaglass cladding (at 1060 nm). The purchased mode field adapters includeda double-clad output fiber having an output fiber having a core diameterof approximately 20 microns, a first cladding having diameter ofapproximately 400 microns, and a second cladding comprising a low indexpolymer. The NA of the core to first cladding of the output fiber isapproximately 0.06 and the NA of the first cladding to the secondcladding of the output fiber is approximately 0.46. The purchased modefield adapters were used in the MOPA architecture shown in FIG. 1. TheMOPA architecture was intended to ultimately be capable of CW outputpowers in the hundreds of Watts. A plurality of such mode field adapterscatastrophically self-destructed during operation of the MOPA in whichthey were used. Inspection after catastrophic failure indicated thatfiber was entirely missing where one would have expected to find a taper(thus Applicant's uncertainly as to the exact structure of the taper, asonly post mortem inspections were made of the purchased mode fieldadapters, and at this point fiber was simply missing). Applicant assumesthat the mode field adapters at least included a tapered section of coreas generally shown in FIG. 2.

Applicant considers the catastrophic failure to occur, at least in part,because of the concentration of energy by the mode field adapter whensubjected to backward propagating optical energy. For forwardpropagating optical energy, that is, optical energy propagating from theinput fiber section 44 to the output fiber section 46, the taperedsection 48 likely slowly (e.g., adiabatically) expands the MFD of thefundamental mode of the SM core of the input fiber so as to avoidexcitation of higher order modes and to match the fundamental mode ofthe MM output optical fiber. The forward propagating optical energy isof low power and the power density decreases even more as the taperedsection expands the MFD. However, under reverse or backward propagation,the opposite can be true—first, the reverse propagating optical energycan be a pulse having a high power density, and second, the taperedsection 48 now serves to concentrate the energy, increasing the powerdensity such that it exceeds the damage threshold of one or morematerials of the mode field adapter. The lack of an outer coating overthe tapered section 48, such that it is essentially “air” clad (or theuse of coating having a lower index of refraction than the cladding 70,such that the coating acts as cladding) tends to promote concentrationof energy by confining any leaked optical energy to the cladding 70.However, brightness conservation means that under reverse propagationsome light must leave the core 68, as the light is being confined to asmaller cross sectional area and brightness cannot be increased by apassive device. Some light therefore leaks into the cladding.Furthermore, the reverse propagating energy may be MM and hence havehigher order modes that have a high NA. The tapered section 48 also morereadily causes the NA of the higher order modes of the backwardpropagating light to exceed that of the tapered core relative to itscladding, leaking optical energy into the cladding, as the angle ofreflection becomes less shallow due to the taper.

Leaked optical energy may propagate in claddings 70 and then 52 until itreaches location 76, wherein the much higher refractive index of thecoating 56 of the Corning HI 1060 input fiber to the silica cladding 52very rapidly strips the light into the coating 56, causing burning ordamage to the coating 56. Damage to the coating 56 can raise the opticalabsorption of the coating 56, further increasing damage. This viciouscycle of damage and attendant increased energy absorption may lead to a“thermal runaway” that very rapidly destroys the mode converter 42. Insummary, Applicant considers that the prior art mode field adapterlikely allows backward propagating optical energy to become tooconcentrated and when stripped from the cladding within the modeconverter, to be stripped in too rapid and localized a manner.

Applicant's experiments were performed with what ITF's productliterature describe as “mode field adapter”, whereas the term “modefield converter” is used in the '630 patent. “Mode field adapter” isused by Applicant as a broader term that includes within its ambit, butis not necessarily limited to, a “mode field converter” that convertsthe mode of input beam to “match” the fundamental mode of a MMamplifier. Accordingly, as the terms are used herein, not every devicethat is a mode field adapter need be a mode field converter, but a modefield converter would be considered a mode field adapter.

Isolators are known in the art. Isolators have non-reciprocal insertionloss and can be used to protect a source, such as a master oscillator,from backward propagating optical energy. An appropriate isolator 63could be positioned between the mode converter 18 and MM power amplifier24 of FIG. 1, and would help protect both the MO 14 and the modeconverter 18. However, this adds an additional and typically expensivecomponent to the MOPA design, raising cost and complexity. Furthermore,fiber pigtailed optical isolators are typically based on reflection andFaraday rotation using bulk materials. Such isolators typically requirecoupling light out of the input pigtail for propagation in free spaceand/or the bulk material and re-coupling light back into the outputpigtail. Accordingly, an isolator can be sensitive to temperaturefluctuations and vibrations, as the fluctuations and/or vibrations canaffect internal optical alignment of the isolator.

FIG. 3 schematically illustrates one embodiment according to theinvention of an optical fiber coupling apparatus for optically couplinga source of optical energy having a selected wavelength and an opticalamplifier and having decreased susceptibility to catastrophic damage tothe source or coupling apparatus from optical energy propagating backfrom the optical amplifier to the source. With proper design, theoptical fiber coupling apparatus 342 may have reduced susceptibility tocatastrophic failure under high power operation as well as be able tomode field convert the fundamental mode of a SM fiber to match thefundamental mode of a MM fiber. Accordingly, in such practices, theoptical coupling apparatus can be used as the mode converter shown inFIG. 1.

The optical coupling apparatus 342 comprises an input optical fibersection 344, typically a SM input fiber, a MM output optical fibersection 346, and a section of fiber 348 between the input optical fibersection 344 and the MM output fiber 346. The fiber section 348 includesat least a core 368 including a taper, as shown in FIG. 3. The cladding370 of the section 348 may also include a taper, as shown in FIG. 3. Theinput optical fiber section 344 comprises a core 350 and a cladding 352,typically a silica glass cladding, disposed about the core 350, as wellas a polymer coating 356 disposed about the cladding 352. The outputfiber section 346 is typically a double clad fiber including a MM core360, an inner cladding, or “pump,” cladding 362 surrounding the core360, and a second cladding 366 surrounding the inner cladding 362. Thesecond cladding 366 can comprise a polymer coating having a refractiveindex that is less than a refractive index of the inner cladding 362.

The optical coupling apparatus 342 can be fabricated by tapering thelarger output fiber 346 to reduce the diameter of the core to the pointwhere the MFD approaches or matches the MFD of the SM input fibersection 344, and the SM input fiber and tapered end are splicedtogether, such as at the location indicated by reference numeral 374.Other techniques may be possible, such as, for example, speciallydesigning the input fiber section 344 such the that the core can begradually expanded as a function of length using thermal techniques tomake a section akin to section 372 wherein the diameter of the coreincreases as shown in FIG. 2.

The optical coupling apparatus 342 of FIG. 3 can include a region 379contactingly disposed about the cladding 370 of the section 348, suchas, for example, along the length indicated by reference numeral 372.The region 379 should comprise an index of refraction that is higherthan an index of refraction of the underlying region (e.g., the cladding370 in FIG. 3), but too large an index difference is avoided. The indexdifference is selected to be small enough to slowly extract the leakingoptical energy such that dissipation occurs over wider area andcatastrophic failure is avoided, or at least the threshold therefore ispushed out to higher power levels. The index difference between theregion and the cladding 370 about which the region is disposed can, invarious practices of the invention, be no greater than 0.035, no greaterthan 0.030, no greater than 0.028, no greater than 0.026, no greaterthan 0.024, or no greater than 0.020. In some practices of theinvention, the index of refraction of the region 379 is no greater than1.49357 no greater than 1.48857, no greater than 1.486, no greater than1.48357 or no greater than 1.47857. References to the foregoing indicesof refraction of the region 379 can refer to the index of refraction asmeasured at 587 nm. Because an absolute value for an index of refractioncan vary to a certain degree with wavelength, it is preferable at timesto refer to the differences between the indices of refraction ofregions, which can be those at a wavelength of operation of the deviceor region. Typically the claddings 352, 362 and 370 are substantiallysimilar (e.g., all comprise silica glass) such that selecting the region370 to have a selected index difference with one of the claddings meansthat substantially the same difference exists with the others of thecladdings. The region 379 need not have the full extent shown in FIG. 3,though it is likely preferable that the region 379 extends to eitherside of location 374 as this is where the backward propagating opticalenergy may be most likely be concentrated and/or escape, especially ifthe optical apparatus includes a splice at location 374.

It is also considered desirable that the region 379 be highly opticallytransparent (e.g., as close to 100% as possible) at the wavelengths oflight to be stripped, which wavelengths would typically include thewavelength of the output light from a fiber laser or optical fiberamplifier. This prevents significant absorption of any stripped light.The highly transparent nature of region 379 should also reduce thethermal stress on the region due to stripping the backward propagatingrogue energy. The region can be comprised of an epoxy having theforegoing optical characteristics. As optical transparency at thewavelength of stripped light may not always be specified bymanufacturers of materials that region 379 can include, data for opticaltransparencies according to UV/VIS spectroscopy, which are oftenspecified, may have to be relied upon in choosing one material overanother. Data from IR spectroscopy would be preferable in cases where IRwavelengths are nearer to the intended operating wavelength of theoptical fiber coupling apparatus. It is also desirable that the regionbe comprised of a material having appropriate thermal properties, suchas good heat conduction and a high degradation temperature. The materialshould not degrade at temperatures well above the temperature at which astandard optical fiber protective coating may degrade (e.g., about 100°C.). Epoxies are available that have thermal degradation temperatures ofat least 200° C. The temperature characteristics should be such as toavoid the “thermal runaway” described above as suspected of contributingto the catastrophic failure.

The optical coupling apparatus 342 can further include another region383 disposed about region 379. The region 383 can, in various practicesof the invention, serve one or more of several purposes. For example,the region 383 can comprise a housing that confines region 379 (e.g., ifregion 379, comprises, for example, a liquid). The region 383 canprovide mechanical support for region 379, such as by comprising a rigidstructure to which region 379 adheres. Neither the region 383 nor theregion 379 need completely surround the fiber of the optical couplingapparatus 342. For example, the optical fiber of the coupling apparatus,including the core and cladding, can lie in a groove cut in a structureof solid metal (e.g., a piece of aluminum) and the region 379 cancomprise an epoxy interposed between the optical fiber and the piece ofaluminum. The epoxy can secure the fiber to the aluminum comprised bythe region 383. The region 383 can have a good thermal conductivity forhelping to remove heat from region 379. For example, the region 383 canbe in thermal communication with the region 379 and can have a thermalconductivity of, for example, in various practices of the invention, atleast 25 Watt/meter-Kelvin (“W/MK”), at least 50 W/MK, at least 75 WM/K,at least 90 W/MK or at least 100 W/MK.

The region 383 can facilitate the safe disposal of optical energystripped by the region 379, and can be contactingly disposed about theregion 379, as shown in the embodiment of the invention illustrated inFIG. 4. The goal is to safely dispose of the stripped rogue opticalenergy and to hence avoid damage to optical apparatus. The region 383can be absorptive, but preferably only to the extent that provision ismade for thermal transfer from the region 383 to avoid overheating.However, it is also possible that the region 383 can have a reflectivitysuch that optical energy is directed out of the region 379, such as awayfrom the optical interconnection apparatus 440 and into the ambientenvironment. For example, in one practice FIG. 3 can represent a topview, looking down, on a groove machined into a piece of aluminumcomprised by region 383, and, as noted above, region 379 can compriseepoxy that adheres the section 372 of fiber to the aluminum. In thisinstance optical energy can be reflected from aluminum and scattered upout of the region 379 (i.e., out of the plane of the page) and into theambient environment. The region 383 can comprise a combination ofabsorptive and reflective properties. Broadly considered, the region 383has a transmittance that is different (e.g., less and usually much less)than that of region 379, as optical energy that is reflected or absorbedby a material is not transmitted. Transmittance generally refers to theratio of the total radiant energy transmitted through and emerging froma body to the total radiant energy incident on the body. Relativetransmittance can be determined by measuring the ratio of incidentoptical energy to transmitted optical energy for a narrow collimatedlight beam normally incident on sample of each of the regions, where thesamples have the same thickness, which should be of the order of athickness used in the region 379 of an optical coupling apparatus.Region 383 can comprise a metal, such as, for example, a pure metal, ora matrix that includes metallic particles, or an alloy of differentmetals. Region 383 can comprise a ceramic or other material, such asgraphite or carbon. Region 383 can include one or more of the foregoingtogether as well as with other materials.

It is noted that “pump dumps” are known in the prior art for providinglocalized stripping of unabsorbed pump light from the inner cladding ofa double clad fiber. In such a pump dump, the second cladding, which istypically a low index coating that provides the inner cladding with ahigh (e.g., 0.46) numerical aperture such that the inner cladding maypropagate pump light, is removed from the double clad fiber along ashort length of the fiber. Along the short length the removed secondcladding is replaced with a material having a higher index of refractionfor stripping the pump light from the inner cladding. The material insuch prior art pump dumps can have high (e.g., 100%) opticaltransmissivity at the wavelength of the pump light and an index ofrefraction difference with the inner cladding that is low, (e.g.,between 0.02700 and 0.02800) as 0.02743, for example. In such a priorart pump dump, neither the core nor the inner cladding of double cladfiber are understood to be tapered. The pump dump is intended to stripresidual pump light that was previously deliberately introduced into thecladding, not rogue light initially propagating in the core and thatsuddenly leaks to the cladding. The material can be an epoxy that cancontact a metal structure, such as an aluminum block, that absorbs pumplight stripped from the inner cladding by the material.

The optical apparatus of FIG. 3 is thus considered to be more robust andto be able to mode field adapt while providing more protection againstcatastrophic failure due to rogue backward propagating pulses of energy.However, the apparatus 342 of FIG. 3 still includes a taper thatinevitably undesirably concentrated backward propagating light. It ispossible that the higher order modes of MM backward propagating lightmay be dissipated over an extended length of the taper, helping withsafe power dissipation. For example, each higher order mode isassociated with a different angle of reflection from the interfacebetween the core 368 and the cladding 370, and the higher the order ofthe mode, the less shallow (compared to the boundary) the angle ofreflection. For backward propagating light, the taper increases theangle (decreases the shallowness) such that the NA of higher order modeswill exceed the NA of the core as determined by the index differencebetween the core and the cladding. Accordingly, optical energy of higherorder modes will more readily enter the cladding than that of lowerorder modes. A practical taper may be somewhat non linear and maydistribute the energy extraction of higher order modes by the claddingover some finite distance, thereby reducing the power density somewhat,such that the low index coating 379 may safely extract the opticalenergy.

However, the handling by a taper of backward propagating light in lowerorder modes, as well as in the fundamental mode, may be of more concern,in that the taper by design has very low loss for such light and willmode adapt the fundamental mode upon backward propagation to concentratethe light until it reaches a power density that cannot be tolerated bythe taper, leading to localized thermal failure of the taper or perhapsdamage to the MO if the concentrated light is delivered to the MO. Thusa more controlled dissipation of higher order mode light, and perhaps aneven deliberately increased and distributed dissipation of light in thefundamental mode so as to avoid concentration, may provide a greatersafety margin regarding avoiding damage in high power operation. Anadditional concern is that precisely tapering the core and the claddingof a fiber is time consuming and can require specialized skills andequipment not widely possessed. Alternative solutions may be of interestthat are simpler to fabricate and that may even provide a wider safetymargin.

Accordingly, FIG. 4 schematically illustrates an alternative embodimentaccording to the invention of a optical fiber coupling apparatus forproviding decreased susceptibility to catastrophic damage to a source orto the optical coupling apparatus from rogue optical energy propagatingback from the optical amplifier to the source.

The optical fiber interconnection apparatus 440 can comprise an inputreference plane 441 located along an input section of optical fiber 444and an output reference plane 451 located along an output section ofoptical fiber 446. As used herein, “forward propagation” of opticalenergy refers to propagation in the direction from the input referenceplane 441 to the output reference plane 451 and “reverse propagation”refers to light propagating in the direction from the output referenceplane 451 to input reference plane 441. The optical fiberinterconnection apparatus 440 can be interposed between a source andpower amplifier so as to forward propagate a seed beam from the opticalsource to the optical power amplifier.

The optical fiber interconnection apparatus 440 can include at least oneintermediate section 448 of optical fiber. The intermediate section 448is intermediate to, and different than other sections of optical fibersuch that there is one or more selected mismatches (e.g., geometricalmismatches and/or numerical aperture mismatches) between the corecomprised by the intermediate section 448 and the cores of the sections(in the embodiment shown in FIG. 4, sections 444 and 446) in opticalcommunication with (e.g., spliced to) the core of the intermediatesection 448.

Each deliberate mismatch, as discussed in more detail below, providesfor a selective and distributed directing of backward propagatingoptical energy from the cores of the fiber to the claddings of thefibers where the mismatched fibers meet (e.g., are spliced). Provisioncan then be made proximate to the splice to safely strip the rogueenergy from the claddings and dispose of the energy. Such provision caninclude first regions disposed about the claddings adjacent the splicesbetween mismatched cores, where the first regions comprise a materialhaving a selected higher index of refraction relative to that of thecladding and high optical transparency for conducting to other regions(e.g., regions comprising a metal) disposed about first regions, wherethe other regions facilitate safe disposal of the rogue, backwardpropagating optical energy. The selected mismatches divert backwardpropagating energy in the higher order modes, as well as in thefundamental mode, which is directed to claddings, stripped and disposed.

As opposed to the mode field adapter, precise tapers, which can bedifficult to make, can be avoided, and instead simple sections ofdifferent, but constant diameter fibers can be spliced together. Inaddition, the use of at least two splices between mismatched fibersdistributes the stripping of rogue backward propagating energy to twoknown locations for enhanced resistance to failure due to excessivepower density at any one location. Backward propagating energy is notconcentrated such that it leaves the core at an unknown locationtherealong; backward propagating energy leaves at the splices whereprovision can be provided adjacent the mismatch, and not necessarilyalong the entire length of the interconnection apparatus, for safelydealing with the rogue energy. The optical fiber interconnectionapparatus is typically much longer than the taper of a mode fieldadapter. For example, where the interconnection apparatus includes threesections, such as in FIG. 4, each of the sections can be about a meterin length (as opposed to a taper, which may extend only overcentimeters).

Although the invention is not limited to any particular number ofsections, in the embodiment shown in FIG. 4, there are three sections ofoptical fiber, namely, the input section of optical fiber 444, theintermediate section 448 and the output section 446. The intermediatesection 448 of optical fiber can be spliced to the input section 444 atplane 449 and to the output section 446 at plane 453. The input section444 can comprise a core 455, a cladding 459 disposed about the core 455,and a region 461, which in the embodiment schematically shown in FIG. 4comprises a standard protective acrylate polymer coating, disposed aboutthe cladding 459. Such standard protective coatings are understood totypically have a refractive index difference of at least +0.45 relativeto the cladding 459. The input optical fiber 444 can be a SM fiber suchthat the core 455 is single-moded at a wavelength of operation of theoptical fiber interconnection apparatus 440, which is typically thewavelength at which the optical source provides, and the opticalamplifier amplifiers, optical energy. For example, the core 455 can besingle-moded, at least in some wavelengths in the range from 1-3 or inthe range 1-2.2 microns, or over the entire span of one or both of theforegoing ranges. The core 455 of the input section 444 can have adiameter D₁ at a location therealong, such as at one or both of thereference planes 441 and 449, and cross sectional area A₁, (not shown),taken perpendicular to the elongate axis of the input section 444 ofoptical fiber, and a numerical aperture NA₁ relative to the cladding459.

The intermediate section of optical fiber 448 can comprise a core 463, acladding 465 disposed about the core 463, and can also include a region467, which in the embodiment shown in FIG. 4 comprises theaforementioned standard protective acrylate polymer coating having anindex of refraction that is greater than an index of refraction of thecladding 465. The core 463 can comprise a diameter D₂ and a crosssectional area (not shown) A₂ and a numerical aperture NA₂ relative tothe cladding 465.

The output section of optical fiber 446 can comprise a core 469, acladding 471 disposed about the core 469, and region 473 disposed aboutthe cladding 471, where the region 473 typically comprises one or moreacrylate polymer coatings. However, the output section of optical fiber446 is typically a double clad, or a “cladding pumped” fiber such thatthe cladding 471 is dimensioned and/or otherwise adapted (e.g., has anoncircular outer circumference to enhance mode mixing) to serve as apump cladding. Accordingly, the acrylate polymer comprised by the region473 typically has a lower index of refraction selected to provide a highNA (e.g., 0.46) of the cladding relative to the region 473, such thatthe cladding 471 can guide the pump light. The core 469 of the outputsection 446 can have a diameter D₃, a cross section area A₃, and anumerical aperture NA₃ relative to the cladding 471 disposed about thecore 469.

There can be a geometrical mismatch between the core 455 of the inputsection 444 and the core 463 of the intermediate section 448. The term“geometrical mismatch” as used herein is used to compare similar regions(e.g., to compare two cores or to compare two claddings) of opticalfiber, and is to be taken as the square of the ratio of diameters of thetwo regions, with the larger diameter being in the numerator, such thatthe geometrical mismatch is always 1 or greater than 1. If a region isshaped, such that it does not have a constant diameter when viewed incross section taken perpendicular to the elongate axis of the fiber, aneffective diameter can be used, where the effective diameter is thediameter that gives the correct cross sectional area for a circle havingthe same cross sectional area that defined by the outer boundary of theshaped area. For example, the geometrical mismatch between the core 455having diameter D₁ and core 463 having a diameter D₂ is (D₂/D₁)² If thediameter D₁ is about 7 microns and the diameter D₂ is about 15 microns,then the magnitude of the geometrical mismatch is (15/7)², or about 4.6.

A simple approximation formula can apply to the transmission of highlyMM light between coupled fibers having cores having different crosssectional areas, when the transmission is considered as from a largercross section fiber to a smaller cross section fiber. In such case, thetransmission factor T_(GM) is given by:

T _(GM)=(A ₂ /A ₁)² (applies only when A₂≦A₁, otherwise T_(GM)=1)

Thus a geometrical mismatch can introduce a non-reciprocal loss for MMlight. In the case of coupling between the core 455 of the input section444 of optical fiber and the core 463 of the intermediate section 448 ofoptical fiber (as well as between the core 469 of the output section 446and the core 463 of the intermediate section 448) backward propagatingMM light will experience, in simple theory, a reduction in transmission,as determined by the factor T_(GM) in each case, because in eachinstance the geometrical mismatches for backward propagating light arefrom cores having larger cross sectional areas to cores having smallercross sectional areas. Forward propagating light will not be affected.The MM light not transmitted across the geometrical mismatch will belost to the claddings of the fiber, as indicate by reference numerals477A in the case of the geometrical mismatch between the cores 463 and455 and by reference numerals 477B, in the case of the geometricalmismatch between cores 469 and 463.

The optical interconnection apparatus 440 can also include selected NAmismatches between the cores of the input and intermediate sections andthe cores of the intermediate and output sections of optical fiber. SuchNA mismatches can also be non reciprocal for MM light, and hence can bearranged according to the invention to also only affect backwardpropagating light. For transmission from core having a numericalaperture NA₁ to a core having a numerical aperture NA₂ that is less thanNA₁, the transmission factor T_(NA) can be approximated by:

T _(NA)=(NA ₂ /NA ₁)² (for NA₂≦NA₁ only, otherwise T_(NA)=1)

The NA mismatches can cause additional rogue energy to be lost to thecladdings of the sections of fiber, as can also be indicated byreference numerals 477A in the case of the NA mismatch between the cores463 and 455 and by reference numerals 477B, in the case of the NAmismatch between cores 469 and 463.

Thus, in one practice of the invention, A₁<A₂<A₃ and NA₁>NA₂>NA₃, whichprovides two mismatches, which are each geometrical and NA mismatches,that are non-reciprocal and affect backward propagating light only.

Thus in the embodiment shown in FIG. 4, in the forward propagationdirection, the geometrical mismatches proceed from smaller crosssectional area cores to larger cross sectional area cores and the NAmismatches are from a higher NA to a lower NA. Accordingly, in thebackward direction of propagation, the opposite holds—the geometricalmismatches proceed from larger cross sectional area to smaller crosssection area cores and the NA mismatches proceed from lower NAs tohigher NAs. As one of ordinary skill in the art can discern, based onthe disclosure herein, the input section 444 can have a lower V numberat the wavelength of operation than that of the intermediate section448, which can in turn have a lower V number than that of the outputsection 446, as V-number is a function of the product of the NA and corediameter.

The geometrical and/or NA mismatches between the sections of opticalfiber of the optical fiber interconnection apparatus of FIG. 4 will alsointroduce loss for the fundamental mode. Generally speaking, the MFD ofa fundamental mode is a function of both the diameter (or crosssectional area) and the NA of the core. Larger diameters and smaller NAsprovide larger MFDs, and concomitantly, smaller diameters and larger NAsprovide smaller MFDs. As the above equation for T_(FM) indicates, amismatch between MFDs reduces the transmission factor T_(FM) between thefundamental modes of the cores of two fibers in optical communication.Generally the MFD of the fundamental mode of a fiber is slightly lessthan the diameter of the core, though the degree to which the MFD isless increases as the fiber diameter increases. For example, consider aSM mode fiber having a fundamental mode MFD of approximately 7 microns(which is a typical fiber on which a MO could be based), and a MM fiberhaving a fundamental mode having an MFD of 24 microns (a goodapproximation for a optical fiber power amplifier based on a large modearea (LMA) fiber having a core having a diameter of 30 microns). Theabove equation for T_(FM) yields a transmission factor T_(FM) forunaided optical communication between such fibers (e.g., butt couplinginstead of interposing a mode field converter therebetween) ofapproximately 0.3, meaning that only 30% of the optical energy in thefundamental mode of the SM fiber makes it into the fundamental mode ofthe MM fiber (and vice-versa).

This low transmission factor T_(FM) is beneficial and desired as tobackward propagating light, given sufficient provisions according to theinvention for safely disposing of such light. However, the loss infundamental mode light is reciprocal, and also applies to forwardpropagating light, which can create at least two disadvantages. First,power output of the MOPA can be reduced because less optical energyreaches the power amplifier in the desired fundamental mode. Thishowever, can be a factor of less importance. Often the power amplifierruns in a saturated state such that the output power of the overall MOPAin the fundamental mode is not linearly related to the input power andaccordingly is not as drastically reduced (e.g., not reduced to 30% ofwhat it could be). Also, pump power can be increased to boost theamplification and thus output power of the power amplifier and hence theMOPA so as to reach a certain desired power output (albeit at reducedoperating efficiency).

A second effect is of more concern: the 70% power mismatch betweenfundamental modes means that 70% of the power in the incidentfundamental mode is available to excite, and does excite, higher ordermodes in the MM fiber. In the backward propagation direction, this isnot of much concern, as the geometric and NA mismatches are such thatthe MM light is stripped, and it is desired to fully attenuate the roguepulse anyway. The forward propagation direction is different—the higherorder modes are not attenuated by the geometric and NA mismatches andproceed to the MM amplifier, where they are readily amplified, degradingbeam quality (reducing the M²) of the output beam of the MOPA.Fortunately, however, as discussed below, this disadvantage can also beaddressed, albeit at the expense of added complexity.

Overall, the insertion loss of the optical fiber interconnectionapparatus can be considered nonreciprocal, that is, different in theforward propagation direction than in the reverse direction. If theinsertion loss is to be defined by specifying that the core of the fiberat the reference plane of interest (i.e., for forward propagation,reference plane 441 and for reverse propagation, reference plane 451)insertion loss is measured by substantially illuminating the entirecross sectional area of the core. Accordingly, higher order modes, ifsupported by the core, are excited. To determine insertion loss thepower present in substantially all core modes propagating at the otherreference plane is measured, and the ratio of this power to total powerlaunched into the core and the other reference plane determines theinsertion loss. Accordingly, in one practice of the invention, theoptical insertion loss of the fiber optical interconnection apparatus isnon reciprocal. It is expected (using the equations noted above andtypical values for core diameters) that the insertion loss can be atleast 0.5 dB, 1 dB, 1.5 dB, 2 dB or even 3 dB higher when measured inthe reverse propagation direction than when measured in the forwardpropagation direction. The factors T_(FM), T_(GM) and T_(NA) areillustrative for conceptual purposes, and though in theory the insertionmeasured (for the appropriate type of light) due to a factor, such asT_(GM), for example, is 10 log T_(GM), in practice the actual insertionloss will vary. One of ordinary skill in the art understands that if itis desired to measure the insertion loss experienced only by thefundamental mode, care must be exercised to ensure that one is notmeasuring higher order mode light.

To safely deal with the backward propagating light directed to thecladdings, the optical fiber interconnection apparatus preferablyincludes one or both of regions 479A and 479B, which comprises a regionhaving a higher index of refraction than the claddings about which theyare disposed, but wherein the index difference between the claddings andthe regions is selected to be sufficiently low to more safely strip theoptical energy from the claddings. Considerations discussed above withregard to region 379 (e.g., index of refraction values, as well as indexdifferences between the region 379 and the cladding about which it isdisposed) of the mode field adapter of FIG. 3 apply to the regions 479Aand 479B of the fiber optical interconnection apparatus 440 as well andare not repeated here.

The optical interconnection apparatus 440 can further include a metal(e.g., aluminum) or other absorptive regions 483A and 483B disposed,respectively, about regions 479A and 479B for safely disposing ofoptical energy stripped by the region 379. Again, considerationsdiscussed in regard with regions 483 of FIG. 3 apply with equal force tothe optical regions 483A and 483B of FIG. 4. Thus the geometrical and/orNA mismatches thus introduce controlled, non-reciprocal loss to backwardpropagating light at two different locations, where specific provision,in the form of regions 479A and 479B and 483A and 483B can more safelydissipate stripped optical energy. For higher order modes, forwardpropagating light is not affected, according to the above approximationsfor T_(GM) and T_(NA).

As shown in FIG. 4, the optical fiber apparatus 440 can simply includesections of spliced optical fiber arranged in series and can be an “alloptical fiber arrangement,” which means, as used herein, that that theoptical communication from the input reference plane to the outputreference plane is predominantly or exclusively via optical fiber, anddoes not, for example, include significant free space communication orthe use of discrete lenses and the like.

The cross sectional area A₁ of the core 455 of the input section 444 ofoptical fiber can remain substantially constant along substantially allof the input section 444 of optical fiber, such as along substantiallyall of the distance between reference planes 441 and 449. Similarly, andindependent of considerations regarding the input section, the crosssectional area A₂ of the core 463 of the intermediate section 448 ofoptical fiber can remain substantially constant along substantially allof the intermediate section 448 of optical fiber, such as alongsubstantially all of the distance between reference planes 449 and 453.Similar considerations can independently apply to the output section ofoptical fiber: the cross sectional area A₃ of the core 469 of the outputsection 446 of optical fiber can remain substantially constant alongsubstantially all of the output section 446 of optical fiber, such asalong substantially all of the distance between reference planes 453 and451. However, one or more of the optical fiber sections could alsoinclude a taper. Such a taper could be used to increase or decrease themagnitude of a mismatch. The distances L₁, L₂, L₃, and L₄ shown in FIG.4 can each be on the order of millimeters. For example, each of L₁, L₂,L₃, and L₄ can be approximately 20 millimeters.

Thus the use of two (or more) splices between three (or more) simplesections of fiber can avoid the specialized requirements in terms of oneor more of skill, time and equipment to fabricate a precise taper.Furthermore, the use of two (or more) geometrical and/or NA mismatchesforces a distributed and controlled leaking of the rogue backwardpropagating energy in such a way that it may be more readily safelydissipated. If necessary, more mismatch locations (splices betweenmismatched fibers) can be added for further distribution of thedissipation of the rogue energy. The use of a plurality of splices canhave another advantage, in that it reduces the overall loss offundamental mode light from the input optical fiber 444 to the outputoptical fiber 446 (as opposed to a splice directly from the inputoptical fiber 444 to the output optical fiber 446).

The optical fiber interconnection apparatus 440 can include geometricalmismatches between claddings as well as between cores. As indicated inFIG. 4, the cladding 459 of the input section 444 can have a diameterD_(1CL), the cladding 465 of the intermediate section 448 can have adiameter D_(2CL), and the cladding 471 of the output section 446 canhave a diameter D_(3CL). In one practice of the invention considered toreduces loss in the forward propagation direction, the magnitude of thegeometrical mismatch between the cladding 459 of the input section 444and the cladding 465 of the intermediate section 448 is less than themagnitude of the geometrical mismatch between the cladding 465 of theintermediate section 448 and the cladding 471 of the output section 446.

In another practice of the invention, the magnitude of the geometricalmismatch between the core 455 of the input section 444 and the core 463of the intermediate section 448 can be less than the magnitude of thegeometrical mismatch between the core 463 of the intermediate sectionand the core 469 of the output section 446. The foregoing is consideredadvantageous when present in conjunction with aforementioned conditionfor the relative magnitudes of the geometrical mismatches between thecladdings.

The optical fiber interconnection apparatus of FIG. 4 can haveadvantages. It can provide a useable and more robust transition from aMO based on a fiber having a smaller cross sectional area core to anoptical fiber power amplifier based on fiber having a core having alarger cross sectional area. Also, the optical fiber interconnectionapparatus can be simpler to fabricate than the mode field adapter ofFIG. 3, and by deliberately introducing loss to the fundamental as wellas higher order modes, can more likely protect the MO from the backwardpropagating energy. However, such benefits are not achieved withoutdrawbacks. For example, as evident from calculation above regarding MFDmismatches, the optical fiber interconnection apparatus of FIG. 4 doesnot convert and match fundamental modes. To continue with the electricalanalogy noted earlier, the effect of the missing mode conversionfunction can be compared with leaving out an impedance matchingtransformer between a radio frequency (RF) signal source and a loadhaving an input impedance that is mismatched to output impedance of theRF signal generator. The two similar penalties are (1) poor coupling ofpower to the load, with attendant loss of power output and (2) the powerthat isn't coupled into the load can cause problems, such as anunacceptably high standing wave ratio between the RF generator and theload.

Accordingly, when input section 444 comprises a SM core the fundamentalmode of the core of input section is not converted so as to match thefundamental mode of the MM output optical fiber 446. Analogous to (1)above, whereas a mode converter can have very low loss in the forwarddirection, due to the adiabatic taper and matching of modes, the opticalfiber interconnection apparatus of FIG. 4 can have significantlyincreased insertion loss in the forward direction (e.g., 1 dB), due tothe reciprocal nature of fundamental mode loss. Analogous to the “otherproblems” of (2) above, the mismatches, even for propagation in theforward direction, launch light into the higher order modes of theintermediate section 448 and/or the output section 446 of the opticalfiber interconnection apparatus. Thus a MM power amplifier receiving itsinput from the optical fiber interconnection apparatus will amplifythese higher order modes and have reduced beam quality. The firstproblem, as noted above, can be of lesser concern, and can be addressedby increasing pump power. Dealing with the second problem—higher ordermode excitation—is now discussed.

With reference to FIG. 5, one embodiment of the invention includes theMO 514, SM optical fiber 516, an optical coupling apparatus, such as theoptical fiber interconnection apparatus of FIG. 4 having a MM outputsection of optical fiber and, as opposed to a MM amplifier, a SMamplifier 527. The SM amplifier, though based in whole or in part on anoptical fiber having an increased core size and hence lower powerdensity for forestalling the onset of nonlinearities, amplifies only thefundamental mode, such that the output optical fiber 528 provides anoutput beam 532 having good beam quality.

Such SM amplifiers are known in the art. For example, U.S. PatentApplication Publication U.S. 2006/0024008 A1, (“'008 application”)published Feb. 2, 2006 and listing Almantas Galvanauskas as inventor,teaches a composite waveguide comprising a central core and at least oneside core helically wound about the central core and in opticalproximity to the central core. The central core may be configured forlarge mode areas, that is, have a large cross sectional area, akin tothat of a MM core of an optical fiber. However, the composite structureof the central core and helical side core can provide efficient andhighly selective coupling between higher-order modes in the central coreand helical side core. Further, the composite structure provides highloss for modes propagating in helical side core and hence imparts highloss onto the coupled higher-order modes of central core. Thus thecentral core of the fiber is effectively SM. The core can be doped withan active material and pumped to provide amplification.

U.S. Pat. No. 6,496,301, (“'301 patent”) issued on Dec. 17, 2002 toKoplow et al., and entitled “Helical Fiber Amplifier” teaches anotherexample of a SM amplifier. The '301 patent teaches winding doped opticalfiber having an increased core size into coils having a carefullyselected radius such that the bend loss provides spatial filtering suchthat higher order modes that would otherwise propagate are highlyattenuated. The attenuation of the fundamental mode remains low. Theoptical fiber, though having a core having an increased cross sectionalarea for reducing susceptibility to nonlinearities, is essentiallysingle mode. In an optical fiber power amplifier based on this techniquehigher order modes are not amplified.

The techniques of the '008 application and the technique of the '301patent have similarities as well as difference. In both techniques, acentral core of the amplifier fiber has an increased cross sectionalarea to reduce power density and forestall the onset of nonlinearities.In the '008 application technique, higher order modes that wouldotherwise exist in such a core are coupled to the side core, whichspirals around the main core. The spiraling, in addition to helpingselectively couple only the higher order modes and not the fundamentalmode, includes a lot of bending of the side core and introduces a highbend loss to those higher order modes such that they are attenuated. The'301 patent bends the core of the amplifier directly, to a criticalradius, that introduces high bend loss only to the higher order modes ofthe central core of the amplifier fiber. The '008 application requires acomplex fiber, but not complicated fixturing to achieve a critical bendradius, whereas the '301 patent uses a simpler fiber design but canrequire more complicated and precise fixturing (as well as some fiberdesign modification).

The SM power amplifier 527 can be based on either of the foregoingtechniques, though the '301 patent technique has seen considerably morepractical application. The SM amplifier can provide an amplified outputbeam 532 having good beam quality. Although the use of the SM amplifier,as opposed to a simpler MM power amplifier, adds complexity, it can alsobe beneficial in that the amount of rogue, backward propagating energyin higher order modes should in theory be reduced due to the attenuationof higher order modes provided by the SM amplifiers of the designsdiscussed above.

Applicant considers that it may be possible to provide yet furtherprotection against damage from rogue backward propagating energy byfurther distribution of the dissipation of the backward propagatingoptical energy. With reference to FIG. 1 (or FIG. 5), the MO 14 (514 inFIG. 5) typically includes a laser cavity comprising, for example, apair of reflectors spaced along a length of optical fiber doped with anactive material, such as a rare earth, for providing light responsive tothe doped section of fiber receiving pump light. Many types ofreflectors are known in the art, including, for example, bulk ordeposited mirrors, which can be dichroic for passing pump energy in anend pump configuration, fiber loop reflectors, and even just the cleavedend faces of optical fibers. However, in many designs a reflectorcomprises a fiber grating written into an optical fiber using actinicradiation, as this technique is now well established in the fiber opticart. The fiber laser cavity typically comprises one “high reflector,”which can be designed to reflect as near 100% of the optical energyincident thereon at the output wavelength of the laser cavity aspossible. The other reflector, often referred to as the “outputreflector” or “output coupler,” can have a reflectivity that istypically considerably less than 100%, such as, for example, 20% orless, so as to transmit a selected amount of optical energy at theoperating wavelength of the fiber laser cavity (e.g., the wavelength atwhich the gratings are the most reflective) and provide an output fromthe fiber laser cavity.

In one practice of the invention, the fiber laser cavity of the MOincludes the optical fiber interconnection apparatus 440 of FIG. 4. Forexample, the optical fiber interconnection apparatus 440 can be situatedwithin the fiber laser cavity of the MO, such as between the reflectors,and shielded, at least partially, from backward propagating energy bythe output coupler, which will at least partly reflect the backwardpropagating rogue energy back from whence it came, where apparently ishas much less of a propensity to do harm (as it was generated there tobegin with).

Accordingly, FIG. 6 schematically illustrates yet another embodiment ofthe invention. The MOPA fiber laser apparatus 612 includes the MO 614optically coupled to the optical fiber power amplifier 627, which can bea SM amplifier, as described above, if a high beam quality output beam632 is desired. The MO 614 can comprise a pair of reflectors, 615 and617, that define a fiber laser cavity 619 that also includes opticalfiber that comprises a rare earth for providing optical gain responsiveto fiber of the cavity 619 being pumped by pump light. The pump lightsource 621, which can comprises a pump laser diode or diodes, can beoptically coupled to the cavity 619, such as via optical fiber coupler625 including a pump leg 623 that receives pump light from the lightsource 621. Many types of pump couplers for coupling pump light tolasers are known in the art. For example, as understood by one ofordinary skill in the art, the optical fiber coupler 625 can comprise atapered fiber bundle or a side coupler. As indicated by the embodimentshown in FIG. 6, the coupling of pump light to pump the cavity can beachieved within the fiber laser cavity 619. The coupler can be arrangedso as to couple pump light substantially only to SM fiber of the fiberlaser cavity 619.

The section of optical fiber 629 can comprise the rare earth forproviding the aforementioned optical gain of the laser cavity 619responsive to receiving the pump light. The section 629 of optical fibercan have a length on the order of meters and can comprise a SM core thatis doped with the rare earth. The reflector 617 can comprise a fiberBragg grating written into the core of the section 639 of optical fiber.The reflector 615 can be highly reflective (e.g., nearly 100%) andcomprise a fiber Bragg grating written into SM optical fiber. Thesection 639 of optical fiber can comprise a core that can be MM and havea higher V number, as well as a lower NA, than the core of the section629 of optical fiber. The reflector 617 can be partially reflective suchthat it transmits output light from the laser cavity 619 to the section661 of optical fiber, which is outside of the laser cavity 619, andwhich can also have a core that is MM and that has a higher V number, aswell as lower numerical aperture, than the core of the section 629 ofoptical fiber. Typically, one or all of the cross sectional area, Vnumber and the numerical aperture of the core of the section 661 ofoptical fiber will be substantially the same as the cross sectionalarea, V-number and numerical aperture of the core of the section 639 ofoptical fiber, though there may be slight differences, as the section639 may more photosensitive to facilitate writing of the fiber Bragggrating comprised by the reflector 617.

The fiber laser cavity 619 can comprise the optical fiber couplingapparatus 633, which can be, for example, the optical fiberinterconnection apparatus 440 of FIG. 4, with reference planes 643 and645 of FIG. 6 corresponding, respectively, to reference planes 441 and451, respectively, of FIG. 4. Accordingly, in this configuration, theoutput reflector 617 shields the coupling apparatus 633 by reflecting atleast a portion of the rogue, backward propagating energy back againtowards the power amplifier 627, as indicated schematically by arrow637. The extra margin of protection from rogue, backward propagatingoptical provided by the configuration shown in FIG. 6 does have a price,however, at least in that the coupling apparatus 633 introducesadditional loss into the laser cavity 619, where it may have a greatereffect on reducing the overall efficiency of the MOPA 612 due to theresonant nature of the fiber laser cavity 619 than in the configurationshown in FIG. 5.

As is evident from the discussion accompanying FIG. 4, the optical fiberinterconnection apparatus 440 can include various sections of opticalfiber (e.g., the input, intermediate and output sections of opticalfiber). In one practice of the embodiment of the MOPA 612, the section629 comprises a core that is SM at the output wavelength of the lasercavity 619, and the input section of the interconnection apparatus alsocomprises a SM core. However, the output section of the optical fiberinterconnection apparatus, as well as the section of optical fiber 639,can comprise cores that are MM at the output wavelength and that havelower NAs, but higher V-numbers, than the section 629 and/or the inputsection of the interconnection apparatus. The output reflector 617 cancomprise a fiber Bragg grating that is written in a MM core.Accordingly, the fiber laser cavity 619 can comprise a wavelength ofoperation at which the laser cavity 619 provides output light viatransmission through the output reflector 617, which, as noted above,can comprise a grating written in the MM core of fiber section 639.

In various practices of the invention, the majority (or even, in somepractices, most) of the optical gain provided by the fiber laser cavity619 can be derived from, for example, optical fiber having a SM core(e.g., optical fiber section 629) or from a section of fiber comprisedby the cavity having a lower (or even, in some practices, the lowest) Vnumber, NA or cross sectional area as compared to another fiber sectioncomprised by the fiber laser cavity 619 (or in some practices, ascompared to all other sections of optical fiber comprised by the fiberlaser cavity 619). In one practice the majority or most of the gain ofthe fiber laser cavity is derived from a section of fiber that meetsmore than one of the foregoing criteria, such as by having a lower coreNA, lower core cross sectional area (and hence a lower V-number) thananother section, or of all other sections, of optical fiber comprised bythe fiber laser cavity 619. Substantially all, or at least a majorityof, the optical gain of the fiber laser cavity 619 can occur within SMfiber of the fiber laser cavity. The fiber laser cavity 619 can includea section of MM optical fiber that is substantially free of the rareearth, or of any rare earth, and the fiber laser cavity 619 can besubstantially free of any MM optical fiber that includes the rare earth,or of any rare earth that provides gain or of any rare earth whatsoever.

The section 661 can comprise a MM core for propagating the output lightfrom the fiber laser cavity 619 for providing optical communication withthe optical fiber amplifier 627. The optical fiber amplifier 627 cancomprise a SM optical fiber power amplifier that is based on opticalfiber having an active core that would normally be MM but that is coiledas taught in the '301 patent. A diameter and the NA of the core of anoptical fiber comprised by the optical fiber amplifier 627 can besubstantially the same as a diameter and the NA of the core of thesection 661 of optical fiber.

As some of the rogue light is expected to be reflected by the reflector617, the optical fiber coupling apparatus 633 may in practice be able tosimply comprise a single geometrical and/or NA mismatch between sectionsof spliced fiber and appropriate provision, as discussed above, forsafely disposing of the rogue light directed into the fiber cladding bythe mismatch. In this instance the reflector 617 helps distribute thehandling of the backward propagating light such that perhaps a secondgeometrical and/or NA mismatch is not as necessary. In other practicesof the invention, the optical fiber coupling apparatus 633 can comprisea tapered section of fiber or a mode field adapter, which may have ahigher margin of protection from unduly concentrating rogue energy dueto the partial refection thereof by the reflector 617. Generallyspeaking, the optical fiber of the fiber laser cavity 619 can comprise afirst length of optical fiber comprising a core having a first crosssectional area and a second length of fiber having a core having asecond cross sectional area that is different than the first crosssectional area and a section of tapered fiber, which may act as anoptical fiber mode field adapter, interposed between the first andsecond lengths of optical fiber for providing optical communicationtherebetween. On the other hand, the fiber laser cavity 619 can besubstantially free of a tapered section of optical fiber, such as, forexample, a section of fiber having a tapered core.

In conclusion, as appreciated by one of ordinary skill, in light of thedisclosure herein, Applicant has thus taught a number of options fordealing with the problem of damage to one or more components of anoptical fiber MOPA apparatus from rogue backward propagating energy. Theoptions vary and reflect various design tradeoffs in terms of simplicity(e.g., number of components), quality of output beam, ease with which ahigh quality output beam can be provided, operating efficiency andmargin of safety from damage from rogue, backward propagating light,etc. The simplest, most elegant option with fewest drawbacks in terms ofbeam quality and operating efficiency may be the tapered mode fieldadapter of FIG. 3, designed to perform as a mode field converter formatching the fundamental mode of an optical fiber MM amplifier, with theoverall laser configured as a MOPA according to FIG. 1. This option,however, may require more expertise and specialized equipment and/or nothave the highest degree of protection from damage due to rogue, backwardpropagating energy. At the other end of the spectrum, the option havingthe highest degree of protection from rogue, backward propagating energyis likely the apparatus of FIG. 6, wherein the interconnection apparatusthat includes deliberate geometrical and NA mismatches for introducingcontrolled and distributed losses of rogue light for both light inhigher order modes as well as fundamental mode is included within thefiber laser cavity. This latter option, however, has drawbacks in atleast complexity and operating efficiency. Higher loss is introduceddirectly in the cavity, where it has the most effect in reducingoperating efficiency. In addition, the lack of mode field adaptingprovides not only the higher loss but excites higher order modes aswell, such that to provide high beam quality from the MOPA apparatus thecavity should provide its output to a more complex SM amplifier. Theoption found to be most useful in a particular application will dependon many factors, including the desired performance specifications (suchas, for example, specifications for power output, operating efficiency,M² and the like), skill level, equipment available, and experiencegained over time in trying different designs in different applicationsto find optimal tradeoffs between performance and protection in variousapplications.

Several embodiments of the invention have been described and illustratedherein. Those of ordinary skill in the art will readily envision avariety of other means and structures for performing the functionsand/or obtaining the results or advantages described herein and each ofsuch variations or modifications is deemed to be within the scope of thepresent invention. More generally, those skilled in the art wouldreadily appreciate that all parameters, dimensions, materials andconfigurations described herein are meant to be exemplary and thatactual parameters, dimensions, materials and configurations will dependon specific applications for which the teachings of the presentinvention are used.

Those skilled in the art will recognize or be able to ascertain using nomore than routine experimentation many equivalents to the specificembodiments of the invention described herein. It is therefore to beunderstood that the foregoing embodiments are presented by way ofexample only and that within the scope of the appended claims andequivalents thereto, the invention may be practiced otherwise than asspecifically described.

It is understood that the use of the term “a,” “an” or “one” herein,including in the appended claims, is open ended and means “at least one”or “one or more,” unless expressly defined otherwise. The occasional useof the terms herein “at least one” or “one or more” to improve clarityand to remind of the open nature of “one” or similar terms shall not betaken to imply that the use of the terms “a,” “an” or “one” alone inother instances herein is closed and hence limited to the singular.Similarly, the use of “a part of,” “at least a part of” or similarphrases (e.g., “at least a portion of”) shall not be taken to mean thatthe absence of such a phrase elsewhere is somehow limiting.

1. An optical fiber apparatus for having an increased optical powerthreshold for avoiding damage to the apparatus, comprising: an opticalfiber mode field adapter having an input and an output, said mode fieldadapter transforming optical energy from a fundamental mode having asmaller mode field diameter at the input to a fundamental mode havinglarger mode field diameter at the output, said mode field adapterincluding: a length of optical fiber comprising a core including a taperwherein the cross sectional area of the core increases; a claddingdisposed about said core for tending to confine light to the core so asto be guided by the core; a region disposed about the cladding, saidregion comprising a material contactingly disposed about said claddingand having an index of refraction that is greater than an index ofrefraction of said cladding by a selected amount, said region strippingoptical energy from said cladding, the selected amount being no greaterthan 0.035; and a second region disposed about and in opticalcommunication with said region, the second region for one or reflectingor absorbing optical energy stripped from said cladding by said region.2. The optical fiber apparatus of claim 1 wherein said selected amountis no greater than 0.03.
 3. The optical fiber apparatus of claim 1wherein said selected amount is no greater than 0.028.
 4. The opticalfiber apparatus of claim 1 wherein said selected amount is no greaterthan 0.026.
 5. The optical fiber apparatus of claim 1 wherein saidselected amount is no greater than 0.022.
 6. The optical fiber apparatusof claim 1 wherein said mode field adapter has a wavelength of operationwherein at said wavelength of operation said mode field adapter issingle mode at said input and multimode at said output.
 7. The opticalfiber apparatus of claim 6 wherein said second region comprises at leastone of a metal or a ceramic.
 8. The optical fiber apparatus of claim 1wherein said input and output each comprise a respective length ofoptical fiber comprising a core, a cladding disposed about said core anda selected region contactingly disposed about said cladding, where saidselected region comprises an index of refraction that is different by apredetermined amount than an index of refraction of said cladding andwherein the predetermined amount corresponding to the input length offiber and the predetermined amount corresponding to the output length offiber are both greater in magnitude than the magnitude of the selecteddifference.
 9. The optical fiber apparatus of claim 8 wherein themagnitude of the predetermined amount corresponding to the input lengthof optical fiber is different than the magnitude of the predeterminedamount corresponding to the output length of optical fiber.
 10. Opticalfiber interconnection apparatus for optically coupling a source ofoptical energy having an output wavelength and an optical amplifier andhaving an increased power threshold for avoiding damage to the source orinterconnection apparatus from rogue optical energy propagating backfrom the optical amplifier to the source, comprising: an optical fiberarrangement having an input reference plane and an output referenceplane, said arrangement comprising at least one intermediate section ofoptical fiber having a core spliced at first and second ends,respectively, to the cores of first and second sections of opticalfiber, there being a first geometrical mismatch and a first numericalaperture mismatch between the core of the intermediate optical fiber andsaid core of said first section of fiber and a second geometricalmismatch and a second numerical aperture mismatch between said core ofsaid intermediate section and said core of said third section of fiber,said mismatches being such that in the forward propagation directionwherein optical energy propagates from the input reference plane to saidoutput reference plane, the geometrical mismatches are from smallercross sectional area cores to larger cross sectional area cores and thenumerical aperture mismatches are from larger numerical aperture coresto smaller numerical aperture cores and in the opposite direction ofpropagation, wherein optical energy propagates from said outputreference plane to said input reference plane, the geometricalmismatches are from larger cross sectional area cores to smaller crosssectional area cores and the numerical aperture mismatches are fromsmaller numerical aperture cores to larger numerical aperture cores. 11.The optical fiber interconnection apparatus of claim 10 wherein saidfirst geometrical mismatch is of a greater magnitude than said secondgeometrical mismatch.
 12. The optical fiber interconnection apparatus ofclaim 10 wherein said first, second and intermediate sections of fibereach include a respective cladding disposed about their respective coresfor tending to confine light to each of the cores, and wherein there isa third geometrical mismatch between the cladding of said first sectionand the cladding of said intermediate section and a fourth geometricalmismatch between the cladding of said intermediate section and thecladding of said second section, and wherein the magnitude of saidfourth geometrical mismatch is greater than the magnitude of said thirdgeometrical mismatch.
 13. The optical fiber interconnection apparatus ofclaim 10 wherein one of said cores to which said core of saidintermediate section is spliced is single mode at a wavelength selectedfrom the range of 1 to 2.2 microns and wherein one of the others of saidcores is multimode at said selected wavelength.
 14. The optical fiberinterconnection apparatus of claim 13 wherein said core of saidintermediate section is multimode at said selected wavelength.
 15. Theoptical fiber interconnection apparatus of claim 10 wherein fiber ofsaid optical fiber arrangement includes a cladding disposed about thecore of the fiber for tending to confine light to the core such that thecore guides light; and wherein said apparatus includes a regioncontactingly disposed about a length of the cladding, the region havingan index of refraction that is higher than an index of refraction of thecladding for stripping light from said cladding.
 16. The optical fiberinterconnection apparatus of claim 15 wherein said region is disposedabout one of said splices of said optical fiber interconnectionapparatus.
 17. The optical fiber interconnection apparatus of claim 15wherein the difference between the index of refraction of the region andthe index of refraction of the cladding is no greater than 0.035. 18.The optical fiber interconnection apparatus of claim 15 wherein saidoptical fiber arrangement includes a coating contactingly disposed abouta different length of the cladding, said coating having an index ofrefraction that is higher than the index of refraction of said region.19. The optical fiber interconnection apparatus of claim 15 comprising asecond region in optical communication with said region, said secondregion comprising a metal that can absorb light stripped from saidcladding by said region.
 20. An optical fiber laser apparatus,comprising: a fiber laser cavity having a wavelength of operation atwhich said fiber laser cavity provides output light, said fiber lasercavity including optical fiber that guides light having the wavelengthof operation, said optical fiber having first and second lengths, saidfirst length of optical fiber having a core having a V-number at thewavelength of operation and a numerical aperture, the second length ofoptical fiber having a core that is multimode at said wavelength ofoperation and that has a V-number that is greater than said V-number ofsaid core of said first length optical fiber at said wavelength ofoperation and a numerical aperture that is less than said numericalaperture of said core of said first length of optical fiber, and whereinat least one of said lengths comprises an active material that canprovide light having the wavelength of operation via stimulated emissionresponsive to said optical fiber receiving pump light.
 21. The opticalfiber laser apparatus of claim 20 wherein said fiber laser cavitycomprises a reflector that can reflect light having said wavelength ofoperation, said reflector transmitting light having said wavelength ofoperation for providing an output from said laser cavity, and whereinsaid second length of optical fiber comprises said reflector.
 22. Theoptical fiber laser apparatus of claim 21 wherein said reflectorcomprises a fiber Bragg grating.
 23. The optical fiber laser apparatusof claim 20 wherein said numerical aperture of said core of said secondlength of optical fiber is no greater than 0.1
 24. The optical fiberlaser apparatus of claim 20 wherein said NA of said second length ofoptical fiber is no greater than 0.07.
 25. The optical fiber laserapparatus of claim 20 wherein said core of said first length of opticalfiber is single mode at said wavelength of operation.
 26. The opticalfiber laser apparatus of claim 20 wherein said first length of fiber isspliced to said second length of fiber.
 27. The optical fiber laserapparatus of claim 20 comprising a third length of optical fiber outsideof said fiber laser cavity, said third length of optical fiber having amultimode core for propagating said output light from said fiber lasercavity; and an optical fiber amplifier in optical communication withsaid third length of optical fiber, said optical fiber amplifiercomprising optical fiber comprising a core comprising active materialfor amplifying optical energy having the output wavelength responsive toreceiving pump light.
 28. The optical fiber laser apparatus of claim 20wherein said fiber laser cavity is substantially free of any opticalfiber having a core that is multimode at said wavelength of operationand that comprises said active material.
 29. The optical fiber laserapparatus of claim 20 wherein said fiber laser cavity comprises a modefield adapter arranged so as to provide optical communication betweensaid first and second lengths of optical fiber, said mode field adaptercomprising an optical fiber comprising a core having a taper.
 30. Theoptical fiber laser apparatus of claim 20 wherein said laser cavitycomprises an optical fiber arrangement for providing opticalcommunication between said first and second lengths of optical fiber,said optical fiber arrangement comprising at least one intermediatesection of optical fiber having a core spliced at first and second ends,respectively, to the cores of first and second sections of opticalfiber, there being a first geometrical mismatch and a first numericalaperture mismatch between the core of the intermediate optical fiber andsaid core of said first section of fiber and a second geometricalmismatch and a second numerical aperture mismatch between said core ofsaid intermediate section and said core of said third section of fiber,said mismatches being such that in one propagation direction for opticalenergy by said optical fiber arrangement the geometrical mismatches arefrom smaller cross sectional area cores to larger cross sectional areacores and the numerical aperture mismatches are from larger numericalapertures to smaller numerical apertures.
 31. An optical fiber laserapparatus, comprising: first and second fiber Bragg gratings forreflecting light having a selected wavelength therebetween so as todefine a fiber laser cavity; said fiber laser cavity comprising opticalfiber, a selected length of said optical fiber comprising a rare earthfor providing optical gain such that said laser cavity provides outputlight having the selected wavelength responsive to said cavity receivingpump light; said optical fiber of said fiber laser cavity comprisingboth multimode and single mode optical fiber arranged for guiding lighthaving the selected wavelength and that is reflected by said fiber Bragggratings, the terms “single mode” and “multimode” being used withreference to the core of the optical fiber at the selected wavelength,said first fiber Bragg grating being comprised by single mode fiber andthe second fiber Bragg grating being comprised by multimode fiber; andwherein said second fiber Bragg grating has a selected transmissivity atsaid selected wavelength for providing the output light from the lasercavity and said fiber laser apparatus comprises a length of multimodefiber delivering the output light from said laser cavity.
 32. Theoptical fiber laser apparatus of claim 31 wherein at least a majority ofthe optical gain occurs within single mode fiber of said fiber lasercavity.
 33. The optical fiber laser apparatus of claim 31 wherein saidlaser cavity is substantially free of any multimode optical fiber thatcomprises said rare earth.
 34. The optical fiber laser apparatus ofclaim 31 comprising a coupler for coupling pump light to said fiberlaser cavity, said coupling being achieved within said laser cavity. 35.The optical fiber laser apparatus of claim 34 wherein said couplercouples pump light substantially only to single mode fiber of said lasercavity.
 36. The optical fiber laser apparatus of claim 31 wherein saidlaser cavity comprises single mode fiber having a core having a firstnumerical aperture and multimode fiber a having a core having a secondnumerical aperture that is less than said first NA.
 37. The opticalfiber laser apparatus of claim 31 wherein said optical fiber of saidoptical fiber laser cavity comprises a section having a core, a claddingdisposed about said core and a material contactingly disposed about saidcladding and having an index of refraction that is higher than an indexof refraction of said cladding by a selected amount, said amount beingno greater than 0.035, said material for selectively stripping lightfrom said cladding; and an optically absorbing region contactinglydisposed about said material for absorbing the stripped light.